O God, who in Thine ineffable Providence hast deigned to send Thy holy Angels to keep watch over us: grant to us Thy suppliant people, that we may always be defended by their protection, and may rejoice in their fellowship forever.
Song of the Angels by Bouguereau (+1905)
The existence of Angels, and the role with men, is made rather clear from the pages of Sacred Scripture. Certainly we recall the particular tasks of St. Raphael with Tobias, St. Gabriel announcing the coming of Christ, and St. Michael in vanquishing the wicked angels. We might also recall a few other verses that give us some further thoughts:
In Exodus 23:20-23 we read:
"20 Behold I will send my angel, who shall go before you, and keep you in your journey, and bring you into the place that I have prepared. 21 Take notice of him, and hear his voice, and do not think him one to be contemned: for he will not forgive when you have sinned, and my name is in him. 22 But if you will hear his voice, and do all that I speak, I will be an enemy to your enemies, and will afflict them that afflict you. 23 And my angel shall go before you, and shall bring you in unto the Amorrhite, and the Hethite, and the Pherexite, and the Chanaanite, and the Hevite, and the Jebuzite, whom I will destroy."
Psalm 91 (90):11-12, likewise, notes:
"11 For he has given his angels charge over you; to keep you in all your ways. 12 In their hands they shall bear you up: lest you dash your foot against a stone."
St. John, in his book of Revelation, Chapter 1:1-5:
"The Revelation of Jesus Christ, which God gave unto him, to make known to his servants the things which must shortly come to pass: and signified, sending by his angel to his servant John, 2 who has given testimony to the word of God and the testimony of Jesus Christ, what things soever he has seen. 3 Blessed is he that reads and hears the words of this prophecy: and keeps those things which are written in it. For the time is at hand. 4 John to the seven churches which are in Asia. Grace be unto you and peace, from him that is and that was and that is to come: and from the seven spirits which are before his throne: 5 And from Jesus Christ, who is the faithful witness, the first begotten of the dead and the prince of the kings of the earth, who has loved us and washed us from our sins in his own blood,"
Further, in Chapter 8 of Revelation we have the observation:
"3 And another angel came and stood before the altar, having a golden censer: and there was given to him much incense, that he should offer of the prayers of all saints, upon the golden altar which is before the throne of God. 4 And the smoke of the incense of the prayers of the saints ascended up before God from the hand of the angel."
Finally, in the Gospel of St. Matthew, 18:1-10, we read:
"At that hour the disciples came to Jesus, saying: Who, do you think, is the greater in the kingdom of heaven? 2 And Jesus, calling unto him a little child, set him in the midst of them. 3 And said: amen I say to you, unless you be converted, and become as little children, you shall not enter into the kingdom of heaven. 4 Whosoever therefore shall humble himself as this little child, he is the greater in the kingdom of heaven. 5 And he that shall receive one such little child in my name, receives me. 6 But he that shall scandalize one of these little ones that believe in me, it were better for him that a millstone should be hanged about his neck, and that he should be drowned in the depth of the sea. 7 Woe to the world because of scandals. For it must needs be that scandals come: but nevertheless woe to that man by whom the scandal comes. 8 And if your hand, or your foot, scandalize you, cut it off, and cast it from you. It is better for you to go into life maimed or lame, than having two hands or two feet, to be cast into everlasting fire. 9 And if your eye scandalize you, pluck it out, and cast it from you. It is better for you having one eye to enter into life, than having two eyes to be cast into hell fire. 10 See that you despise not one of these little ones: for I say to you, that their angels in heaven always see the face of my Father who is in heaven."
St. Michael the Archangel by Guido Reni.
St. Thomas Aquinas, in his Summa Theologiae, Pars I, Question 50-64, spends some time discussing the nature and function of angels, which you can find here: Summa Theologiae, Prima Pars (I)
St. Thomas Aquinas specifically discusses the Guardian Angels, in the Prima Pars, Question 113, Article 4, which I present here in full:
Article 4. Whether angels are appointed to the guardianship of all men?Objection 1. It would seem that angels are not appointed to the guardianship of all men. For it is written of Christ (Philippians 2:7) that "He was made in the likeness of men, and in habit found as a man." If therefore angels are appointed to the guardianship of all men, Christ also would have had an angel guardian. But this is unseemly, for Christ is greater than all the angels. Therefore angels are not appointed to the guardianship of all men.
Objection 2. Further, Adam was the first of all men. But it was not fitting that he should have an angel guardian, at least in the state of innocence: for then he was not beset by any dangers. Therefore angels are not appointed to the guardianship of all men.
Objection 3. Further, angels are appointed to the guardianship of men, that they may take them by the hand and guide them to eternal life, encourage them to good works, and protect them against the assaults of the demons. But men who are foreknown to damnation, never attain to eternal life. Infidels, also, though at times they perform good works, do not perform them well, for they have not a right intention: for "faith directs the intention" as Augustine says (Enarr. ii in Ps. 31). Moreover, the coming of Antichrist will be "according to the working of Satan," as it is written (2 Thessalonians 2:9). Therefore angels are not deputed to the guardianship of all men.
On the contrary, is the authority of Jerome quoted above (Article 2), for he says that "each soul has an angel appointed to guard it."
I answer that, Man while in this state of life, is, as it were, on a road by which he should journey towards heaven. On this road man is threatened by many dangers both from within and from without, according to Psalm 141:4: "In this way wherein I walked, they have hidden a snare for me." And therefore as guardians are appointed for men who have to pass by an unsafe road, so an angel guardian is assigned to each man as long as he is a wayfarer. When, however, he arrives at the end of life he no longer has a guardian angel; but in the kingdom he will have an angel to reign with him, in hell a demon to punish him.
Reply to Objection 1. Christ as man was guided immediately by the Word of God: wherefore He needed not be guarded by an angel. Again as regards His soul, He was a comprehensor, although in regard to His passible body, He was a wayfarer. In this latter respect it was right that He should have not a guardian angel as superior to Him, but a ministering angel as inferior to Him. Whence it is written (Matthew 4:11) that "angels came and ministered to Him."
Reply to Objection 2. In the state of innocence man was not threatened by any peril from within: because within him all was well ordered, as we have said above (95, 1,3). But peril threatened from without on account of the snares of the demons; as was proved by the event. For this reason he needed a guardian angel.
Reply to Objection 3. Just as the foreknown, the infidels, and even Anti-christ, are not deprived of the interior help of natural reason; so neither are they deprived of that exterior help granted by God to the whole human race--namely the guardianship of the angels. And although the help which they receive therefrom does not result in their deserving eternal life by good works, it does nevertheless conduce to their being protected from certain evils which would hurt both themselves and others. For even the demons are held off by the good angels, lest they hurt as much as they would. In like manner Antichrist will not do as much harm as he would wish.
Angel comforting Jesus by Bloch.
The Old Catholic Encyclopedia, also, has a good article on the subject of the Holy Angels, in General, Catholic Encyclopedia: Angels, and also one on the Guardian Angels, in particular, Catholic Encyclopedia: Guardian Angels.
Let us never forget the reality and significance of these angelic persons -- nor cease to guard against the fallen of their nature, the demons. Surely this is a part of living, and dying, well!